unix: adjust Statfs_t member types on freebsd/arm

CL 223698 changed the types of the Fstypename, Mntfromname and Mntonname
fields of type Statfs_t on freebsd/{386,amd64,arm64}, but didn't do so
on freebsd/arm. Since reverting that change now would probably cause
more harm than good, at least make the type consistent on all supported
freebsd/$GOARCH combinations.

Change-Id: I66ba3cdb0290177898237a47d305abb9adc97954
Reviewed-on: https://go-review.googlesource.com/c/sys/+/239060
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/ztypes_freebsd_arm.go

@@ -125,9 +125,9 @@ type Statfs_t struct {
 	Owner       uint32
 	Fsid        Fsid
 	Charspare   [80]int8
-	Fstypename  [16]int8
-	Mntfromname [1024]int8
-	Mntonname   [1024]int8
+	Fstypename  [16]byte
+	Mntfromname [1024]byte
+	Mntonname   [1024]byte
 }
 
 type statfs_freebsd11_t struct {